I'm trying to describe the lines you see on a person's body, particularly the abdomen. When someone has very muscular defined six-pack abs, you can clearly see deep narrow lines in between their abs. How do you describe/call these lines?

Noun:
1. A trench cut in the soil, as when plowed in order to plant a crop.Don't walk across that deep furrow in the field.
2. A deep wrinkle in the skin of the face, especially on someone's forehead.When she was tired, a deep furrow appeared on her forehead.

Verb:
1. To make (a) groove, a cut(s) in (the ground etc.).Cart wheels can furrow roads.

'Cut' and 'groove' I normally associate with sharp contours and not with body shapes, but if that's what they call it in the body-building community, then that's what it is. I would rather use 'crease' (but that conjures up folds of fat).
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MitchJan 17 '13 at 17:56

As explained in coleopterist's answer, cut and groove (as in deeply cut and well-grooved) appear to be the terms of art for describing the deep narrow lines of a well-defined abdominal six-pack.

Some words that address the opening question (“What's the word for a deep narrow line?”) include
• slit, “A narrow cut or opening; a slot”
• slot, “A narrow depression, perforation, or aperture...”
• crease, “A line or mark made by folding or doubling any pliable substance; hence, a similar mark, however produced”
• crevice, “A narrow crack or fissure, in a rock or wall”
• crevasse, “(figuratively) A discontinuity or “gap””
• crack, “A narrow opening” or “A thin and usually jagged space opened in a previously solid material”

Note that several of these have well-known vulgar or slang meanings (besides the above senses) and cannot be used straightforwardly for reference to abdominal lines. However, crease may work, if for any reason you want an alternative to cut or groove.

Since body-builders' speak of being cut seems to come from that, one could say cuts but it sounds quite wrong to me.

Groove strikes me a relatively sexy (it sounds like a nice word, even though I don't find the "cut" look attractive myself), furrow more distanced, and perhaps trench if someone found the look strongly unappealing. ("He flexed and strained, deepening the trenches of his over-muscled abdomen" sounds pretty yucky, no?).